This invention relates to an extrusion die employed in the manufacture of webs of thermoplastic polymeric materials and to manufacture of such webs using that extrusion die.
In the commercial manufacture of extruded thermoplastic webs, one serious deficiency in product quality has been nonuniform and excessively thickened edges. Webs will, hereinafter, be called "films" and will be considered to include films, sheets, ribbons, and any thin, shaped, article of indefinite length and large lateral extension. Extrusion of films is accomplished by forcing molten thermoplastic material through an orifice or die slit which has lips spaced apart to make a film of the desired thickness. The spacing of the lips is at least equal to, and sometimes as much as ten times or more, the desired film thickness. The molten extrudate is generally deposited on a chilled, moving, surface where it is frozen into film form.
From the die lips to the chilled surface, the edges of the extrudate thicken due to various forces including the surface tension of the molten material and the speed of the chilled, moving surface relative to the extrusion velocity of material being extruded. The thickened material is known as a "bead" and, in further processing, the bead edge must be trimmed off and discarded. The bead, thus, represents wasted material unless recycled and, in any event, represents a reduction in yield. While some bead thickening may be desirable to facilitate handling of the film, in uncontrolled film extrusion processes the beads may be as much as 2 to 20 times as thick as the unthickened film and thickening over about 1.5 times is generally taken to be undesirable.
The die of this invention is useful for extruding films of a wide variety of thermoplastic materials including polyolefins such as, polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, vinyl polymers such as polyvinyl chloride, and the like. It has been found to be especially useful in the manufacture of films of polyethylene terephthalate.
In U.S. Pat. No. 3,107,191, the extruded film width and bead thickness are controlled by means of inserts located between the die lip surfaces at the ends of the die and shaped such that the inward edges of the inserts diverge slightly approaching the exit of the slit. The patent discloses that this divergence of the inserts causes the film edges to be of the same thickness as interior portions of the film. The thickness of film which can be manufactured by the means of this device cannot be adjusted except by changing to inserts of adjusted thickness.
In U.S. Pat. No. 3,829,274 the width of extruded film is adjusted by means of so-called deckle bars which are placed in the die outlet, where die lip surfaces are parallel, to completely block the flow of molten material. The use of two or more deckles in each molten flow stream is disclosed to permit adjustment of edge bead thickness by adjustment of the relative locations of the deckles. Adjustment of the die outlet slit, and, therefore, film product thickness, is limited by the fact that the edge-starving deckles are bars of small diameter and must remain in contact with both die lip surfaces along the complete length of the deckle.
U.S. Pat. Nos. 3,694,132; 3,706,518; and 3,711,235 all disclose the use of external deckle plates and internal deckle rods as an aid to adjusting the width and edge bead thickness of extruded film. The deckle rods are disclosed to have a variety of configurations but all must be in constant sealing contact with each die lip surface in the die outlet.
U.S. Pat. No. 2,982,995 also discloses use of deckle rods to reduce edge bead thickness. That patent discloses deckle rods which are fitted into holes drilled in the die body along the path of extrusion and which deckle rods have inward-projecting ends which have been cut in a flat slant. The flat slant, it is disclosed, permits some control of molten flow by virtue of the variable amount of interference to flow realized by twisting the deckle in its hole.
It has been found that any internal die configuration which causes "dead space" where molten material can stagnate will cause degradation of the material so stagnated. Thus, a die configuration with sharp discontinuities, such as steps on which deckles rest or cylindrical cavities into which deckle rods fit or crevices between deckle rods and inner walls of the die body, is a die configuration which results in some degree of material degradation. The situation is aggravated in dies with deckles extending inward from the die end for any substantial distance.
In addition, it has been found that restrictions in flow which cause a turbulence result in striations in the extruded film product. Thus, barrier or deckle configurations which are irregular or unstreamlined result in continuous marking along the edge of the extruded film.